Energy question of the week: Can modern freight ships sail using wind power?

Rising fuel costs are urging shipowners all over the world to find ways to deliver cargo across the seas more economically. In the 1920s, the German Aerospace Center's predecessor institute developed Flettner rotors. Some 80 years later, a cargo ship is again sailing with rotating cylindrical sails. But is this change worth it?

Cylinder rotors should reduce marine diesel consumption by a third

Because of the lack of space and the extra crew required, classic masts and rigging have no future on modern container ships. However, the Hamburg company SkySails has already demonstrated that a steerable spinnaker set forward of the ship's bow can save fuel. And today, a cargo ship is sailing with rotating cylinder sails. On the deck of the 'E-Ship 1' there are four 25-metre high Flettner rotors that will power the 130-metre long freighter on its test voyages between Emden and Borkum. The developers, wind turbine manufacturer Enercon, intend to prove that Flettner rotors can save about a third of the vessel's diesel consumption.

"I am convinced that this ship is the first serious step towards the transformation of energy in shipping," says E Ship project manager Rolf Rohden. He and his team are awaiting the test voyages with great excitement. With its maximum speed of 17.5 knots (32.4 kilometres per hour), Enercon intends to use the 'E Ship 1' to transport its wind turbines to customers overseas. The main power for the 22.5-metre wide ship still comes from its conventional engines, each of which develops an output of 3500 kilowatts. But, on long journeys, the 'E Ship 1' could save about one third of its diesel.

The technology behind the 'E-Ship 1' was first developed in the 1920s. With the rotating sails invented by Anton Flettner, a crosswind streaming past a revolving cylindrical sail causes a zone of low pressure to form on the forward side of the cylinder. As a result, the whole ship is pulled forward. Flettner was not the only one to be involved in the invention of this new form of ship propulsion. Several years earlier, Ludwig Prandtl, the Director of the Aerodynamic Test Institute at Göttingen (AVA), had laid the technical foundations. At AVA, the oldest predecessor institute of the modern-day German Aerospace Center, he experimented with rotating cylinders in order to improve the lift of aircraft wings. Prandtl, the 'father of airflow research', however, saw no practical use for such cylinders. It was only later that Flettner suggested using them for ship propulsion.

After numerous wind tunnel tests of rotors at AVA, in 1924 the 'Buckau' sailed on a number of test voyages with two Flettner rotors and, in 1926, even crossed the Atlantic. But because coal-fired power plants being cheap and efficient back then, and introduction of diesel engines, innovative sail propulsion sank into oblivion. It was not until the early 1980s that the French oceanographer Jacques-Yves Cousteau had the research ship 'Alcyone', which is equipped with two Flettner rotors, constructed.

Active on the high sea despite high superstructures

The rotors, which are highly visible, are not the only high-tech components on the 'E-Ship 1'. The underside of the ship is coated with silicone-based paint to reduce losses through friction by creating a very smooth surface. With their long experience in the field of fluid mechanics, the Enercon engineers have optimised the design of the rudders and propellers. On the test voyages, the ship inspection authority, Germanischer Lloyd, will examine and report on all components and – if successful – will confirm the seaworthiness of the freighter. After certification, the wind turbine manufacturer will transport gondolas and rotor blades for up to 20 wind turbines overseas with the 'E-Ship 1'.

The DLR Energy question of the week in 'The future of energy' Year of Science

The Federal Ministry of Education and Research (BMBF) has given the Year of Science 2010 the motto 'The future of energy'. For this reason the science journalist Jan Oliver Löfken will this year answer a question on the subject of energy in his blog each week. Do you have a question about how our energy supply might look in the future? Or do you want to know, for example, how a wave power plant works and how it can efficiently generate electricity? Then send us your question by email. Science journalist Jan Oliver Löfken will investigate the answers and publish them each week in this blog.

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Energy journalist Jan Oliver Löfken writes among other things, for the Technologie Review, Wissenschaft aktuell, Tagesspiegel, Berliner Zeitung and P.M. Magazin on issues involving energy research and industry. For DLR, he answered the Energy question of the week during the Year of Energy 2010.
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